1. Field of the Invention
The present invention relates to a CT (Computed Tomography) scanner which obtains tomographic images by CT reconstruction, and a control method for the scanner.
2. Description of the Related Art
In general, in a CT scanner, a radiation source and a two-dimensional radiation sensor are placed to face each other through an object, and the object is relatively rotated in the radiation emitted from the radiation source. The scanner obtains tomographic images of the object by performing CT reconstruction of projected images of the object which are obtained from various directions during this rotation.
This CT reconstruction requires parameters associated with the position and tilt of the rotation axis of the object relative to the radiation source and the two-dimensional radiation sensor. The scanner cannot obtain high-quality tomographic images without accurately grasping these parameters.
Under the circumstance, as a method of estimating the relative position and tilt of the rotation axis of an object, there is known a method of performing estimation by using a three-dimensional phantom which has a series of cell structures and whose dimension is known in advance based on projected images obtained by CT-scanning the three-dimensional phantom (see Japanese Patent No. 3698167). There is also known a method of performing estimation by generating sinograms from projected images obtained by CT-scanning an object and using the symmetry of the sinograms (see Japanese Patent No. 3846577).
The method using a three-dimensional phantom, however, requires a special three-dimensional phantom, and hence requires scanning of the three-dimensional phantom independently of scanning of the object. In addition, there is no guarantee that the position and tilt of the rotation axis of the object at the time of scanning will perfectly coincide with those of the three-dimensional phantom at the time of scanning. Furthermore, the manufacturing accuracy of a three-dimensional phantom may affect the estimation accuracy of optimal parameters.
The method using sinograms is designed to estimate optimal parameters in the coordinate system of a projected image. It is possible that an estimation error may affect the quality of a tomographic image after reconstruction.